Under the term cisterno-medullary anomalies are included several disorders: bony and nervous malformations, arachnoiditis of the posterior fossa. As they are frequently associated, a thorough investigation, both anatomical and dynamic, is a prerequisite to any therapeutic attempt. Along with causing damage to the neuraxis, these anomalies interfere with the dynamics of the CSF and may lead to the development of a communicating syringomyelia, whatever the theory proposed. The presenting symptoms are varied, and diagnosis should be accordingly suspected. Of fundamental importance are instrumental investigations. A complete evaluation is in most of the cases obtainable with: bone X-rays, air-myelography (or "bulle"), intrathecal ou ventricular radio-isotope scan. Surgery is the only treatment. The aim is both decompression of the neural structures and restoration of a normal CSF dynamic flow. Opening of the posterior fossa is successful in the case of developmental abnormalities, But it seems to prove a failure when the chief anomaly is arachnoiditis. In such cases, ventricular drainage alone may be followed by improvement. It appears from this that the problem is twofold: the technical problem of the drainage, and the pre-operatory diagnosis of a posterior fossa arachnoiditis.